Optically interface electrically controlled devices

ABSTRACT

The present invention presents devices and methods for localized control and transport of excitons as well as separate processing of holes and electrons in a device with an optical input and an optical output. In an embodiment of the invention, an optoelectronic device includes a coupled or wide quantum well structure. A localized gate is arranged over a region of the coupled or wide quantum well structure and a semiconductor barrier layer. A optical input and optical output are arranged over other regions of the coupled or wide quantum well structure that are separated by the gate electrode region. The coupled or wide quantum well structure is dimensioned and formed from materials that create a nonzero distance d between the separated electron and hole of an exciton formed in response to the input. The flow of excitons or separated electrons and holes between the optical input and optical output can be controlled by a voltage potentials applied to the localized gate electrode, optical input, and output gates.

FIELD OF THE INVENTION

A field of the invention is opto-electronics. An exemplary application of the invention is electronic processing with optical communication.

BACKGROUND

Conventional integrated electronics make use of electrical charge to produce electronic effects. Gates control electrical carriers, i.e., holes or electrons in semiconductor devices. Semiconductor devices remain the basis for electronics devices.

Optoelectronic circuits offer the opportunity to make use of the speed of optical signal communication. Optical communications are also less likely to be influenced by interference than electrical signal communications. Typically, optoelectronics finds use in signal communication, such as in optical fiber networks. Optical communications are also used, for example, to communicate information between devices, such as optical communications used in providing audio signals between components in home theatre systems. In conventional optoelectronic devices and systems, interconnects between electronic circuits and optical transceivers are required. Optical signals are not directly processed, but instead are produced when the electrical signals are converted and vice versa.

Conventional optoelectronic devices are based upon a mutual relation between local optical absorption or emission and photocurrent or electric field. This is used in some devices to create electrical current in response to absorbed photons, e.g., in photo sensors. This is also the basis for emission of photons in response to applied energy, which is the basis for semi-conductor lasers, light emitting diodes and other devices.

An exciton is a bound state of an electron and a hole, i.e., a Coulomb correlated electron-hole pair. Exciton formation occurs in a semiconductor when a photon is absorbed, causing an electron to jump the bandgap into the conduction band. The exciton results from the binding of the electron with its hole. Lifetimes and energy of the excitons has been an area of fundamental physics research for a significant time. Researchers have devoted considerable study to the lifetimes and energies of excitons in bulk semiconductor materials.

Coupled quantum well structures in bulk materials have been used to control the lifetime and energy of excitons. The excitons can be created throughout a bulk material or in a quantum well and lifetimes can be controlled, for example, by the application of an electric field. A review of the past work in coupled quantum wells and the studies of lifetimes of excitons is found in “Condensation and Pattern Formation in Cold Exciton Gases in Coupled Quantum Wells”, L. Butov, J. Phys.: Condens. Matter 16 (2004) R1577-R1613.

The present inventor has previously demonstrated control of indirect exciton energy and overlap between the electron and hole wave functions, which results to change of the absorption and emission rate, by applying voltage to a gate over the entire area of the coupled quantum well planar semiconductor structure. The exciton and electron spin relaxation rates were predicted to be determined by the overlap between the electron and hole wave functions in M. Z. Maialle, E. A. de Andrada e Silva, and L. J. Sham, Phys. Rev. B 47, 15776 (1993). As an example, a change of the exciton-emission rate by 10⁴ times, which results to change of the exciton and electron spin relaxation time by 10⁸ times, as well as change of the exciton energy by 40 meV at gate voltage V=1.6 V were demonstrated in Butov et al., “Photoluminescence Kinetics of Indirect Excitons in GaAs/AlGaAs Coupled Quantum Wells,” Phys. Rev. B 59, 1625 (1999).

The electric field in the z direction has been controlled by an external gate voltage V_(g). applied over an area of the bulk semiconductor materials including a coupled quantum well. At low V_(g) (direct regime), the spatially direct exciton is the lowest energy state, while at high V_(g) (indirect regime) the indirect exciton composed of electron and hole in different layers is the lowest energy state. The transition from the direct to the indirect regimes is determined by the ratio between the one-particle symmetric-antisymmetric splittings and the exciton binding energies. For a given coupled quantum well sample, this ratio and the direct-to-indirect crossover can be controlled by magnetic fields. See, Butov, et al. “Direct and indirect magnetoexcitons in symmetric InGaAs/GaAs coupled quantum wells,” Phys. Rev. B 52, 12153 (1995). Butov, et al, “Magneto-optics of the spatially separated electron and hole layers in GaAs/AlGaAs coupled quantum wells,” Phys. Rev. B 60, 8753 (1999).

SUMMARY OF THE INVENTION

The present invention presents devices and methods for localized control and transport of excitons as well as separate processing of holes and electrons in a device with an optical input ad an optical output. In an embodiment of the invention, an optoelectronic device includes a coupled or wide quantum well structure. A localized gate electrode is arranged over a region of the coupled or wide quantum well structure and a semiconductor barrier layer. A optical input and optical output are arranged over other regions of the coupled or wide quantum well structure that are separated by the gate region. The coupled or wide single quantum well structure is dimensioned and formed from materials that create a nonzero distance d between the separated electron and hole of an exciton formed in response to the input. The flow of excitons or separated electrons and holes between the optical input and optical output can be controlled by a voltage potentials applied to the localized gate electrode, optical input, and output gates.

In a method of the invention, photons are absorbed at an input of the optoelectronic device to photo excite electrons and holes and create excitons. A local energy barrier is created with an electric field to control indirect excitons and exciton flow. A voltage pulse is applied to an output drain, increasing exciton recombination to convert excitons to photons produce an optical readout. Methods and devices of the invention permit the digitally processing of separated electrons and holes followed by subsequent recombination of processed electrons and holes to produce a light output.

Another embodiment of the invention is an exciton opto-spin-electronic field effect transistor. The optoelectronic transistor includes an optical input source and an optical output drain. A coupled or wide-quantum well structure creates spin-polarized excitons using circularly polarized light. Spin-polarized excitons travel to the optical drain under control of a gate voltage. The gate voltage controls the spin, and can produce a readout to the optical output drain in response to a voltage pulse that increases the exciton recombination rate to convert the exciton spin state to the photon polarization and thereby produce an optical readout of the exciton spin state at the optical output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate band diagrams of gated quantum well semiconductor devices of the invention;

FIGS. 1C and 1D illustrate the control of indirect exciton energy by gate voltage respectively for the different devices of FIGS. 1A and 1B;

FIG. 1E illustrates control of the recombination rate of indirect excitons by gate voltage;

FIG. 2A illustrates a preferred embodiment gate controlled device of the invention, which can be referred to as an optoelectronic transistor;

FIG. 2B illustrates exciton control via voltage in the FIG. 2A device;

FIG. 3 schematically illustrates an example of integrated circuit devices of an embodiment of the invention, including a two dimensional array of exciton circuits with many optical inputs and outputs and elaborated gate electrode architecture;

FIGS. 4A-4E illustrate separated electron and hole processing according to an embodiment of the invention;

FIG. 5 illustrates a preferred embodiment integrated circuit device for separated electrons and holes.

INVENTION DESCRIPTION

The present invention presents devices and methods for localized control and transport of excitons as well as separate processing of holes and electrons in a device with an optical input and an optical output. In an embodiment of the invention, an optoelectronic device includes a coupled or wide quantum well structure. A localized gate electrode is arranged over a region of the coupled or wide quantum well structure and a semiconductor barrier layer. A optical input and optical output are arranged over other regions of the coupled or wide quantum well structure that are separated by the gate region. The coupled or wide quantum well structure is dimensioned and formed from materials that create a nonzero distance d between the separated electron and hole of an exciton formed in response to the input. The flow of excitons or separated electrons and holes between the optical input and optical output can be controlled by a voltage potentials applied to the localized gate electrode, optical input, and output gates.

In a method of the invention, photons are absorbed at an input of the optoelectronic device to photo excite electrons and holes and create excitons. A local energy barrier is created with a voltage potential applied to the gate electrode to control indirect excitons and exciton flow. A voltage pulse is applied to an output drain, increasing exciton recombination to convert excitons to photons produce an optical readout. Methods and devices of the invention permit the digitally processing of separated electrons and holes followed by subsequent recombination of processed electrons and holes to produce a light output.

In preferred devices of the invention, light input is accepted and processed light output is produced. The device achieves local control of exciton or e-h fluxes and recombination by gates electrodes.

Another embodiment of the invention is an exciton opto-spin-electronic field effect transistor. The optoelectronic transistor includes an optical input source and an optical output drain. A coupled or wide quantum well structure creates spin-polarized excitons using circularly polarized light. Spin-polarized excitons travel to the optical drain under control of a gate voltage. The gate voltage controls the spin flux. A readout to the optical output drain can be produced in response to a voltage pulse that increases the exciton recombination rate to convert the exciton spin state to the photon polarization and thereby produce an optical readout of the exciton spin state at the optical output.

Embodiments of the invention provide optoelectronic circuits that unify electronic operations and optical communications. Preferred embodiment circuits are fully integrated in a chip and electronic operations are directly coupled or wide to optical inputs and outputs. Preferred embodiment monolithically integrated optoelectronic circuits are scalable and allow parallel processing with many optical inputs and outputs.

Embodiments of the invention provide methods of in-plane carrier separation through a laterally modulated gate voltage for integrated optoelectronic circuits. Preferred embodiment methods allow matching integrated electronic circuits with optical inputs and outputs without interconnects. Methods and devices of the invention permit scalable monolithically integrated optoelectronic circuits with parallel processing and many optical inputs and outputs.

An embodiment of the invention is an optical transistor that accepts photons as input, creating an electron-hole pair in the material. The electron-hole pair is prevented from recombining by application of electric fields, enabling the processing of the pairs separately through traditional digital logic. After application of digital logic, electric fields are removed to permit the electron-hole pair(s) to recombine, emitting a photon that can then be routed via traditional optical communication channels. The preferred embodiment transistor can be integrated with other devices in a single IC chip that works in both the optical and electronic domains, reducing the form factor, cost and interconnect complexities of conventional two domain solutions.

Embodiments of the invention include an electron-hole optoelectronic transistor. The invention also includes an exciton optoelectronic transistor. The invention also provides integrated optoelectronic circuits with optical inputs and outputs and electronic operations using separated electrons and holes. The invention also provides fully integrated optoelectronic circuits with optical inputs and outputs and electronic operations using excitons. The invention also provides methods of electronic operations on photons using electrons and holes as intermediate media, and methods of electronic operations on photons using excitons as intermediate media.

An exemplary embodiment of the invention is an electron-hole opto-spin-electronic field effect transistor. Another exemplary embodiment of the invention is an exciton opto-spin-electronic field effect transistor. Another exemplary embodiment of the invention is an integrated opto-spin-electronic circuit with polarized photons at inputs and outputs and electronic operations using separated spin-oriented electrons and holes, referred to as an electron-hole opto-spin-electronic integrated circuit. Another exemplary embodiment of the invention is an integrated opto-spin-electronic circuit with polarized optical inputs and outputs and electronic operations using excitons, referred to as an exciton opto-spin-electronic integrated circuit. The invention also includes methods of electronic operations on polarized photons using electrons and holes as intermediate media and methods of electronic operations on polarized photons using excitons as intermediate media.

Preferred embodiments will now be discussed with respect to the drawings, which may be presented schematically but will be fully understood by artisans with reference to the description. Broader aspects of the invention will be apparent to artisans from the detailed description.

Optoelectronic Devices with Excitons.

Optoelectronic devices of a first embodiment operate with indirect excitons in coupled or wide quantum wells. An indirect exciton is a bound pair of an electron and a hole separated in different quantum wells or on opposite sides of a quantum will with a nonzero distance d. FIGS. 1A and 1B illustrate band diagrams of gated quantum well semiconductor device of the invention including a localized gate electrode for control of excitons. In the example FIG. 1A device, AlAs and GaAs coupled quantum wells are dimensioned to maintain an indirect exciton, with a bound hole and electron in the separate quantum well layers. In the example FIG. 1B device, two coupled GaAs quantum wells are separated by a thin barrier layer, the layers and the barriers are dimensioned to maintain an indirect exciton, with a bound hole and electron in the separate quantum well layers.

FIGS. 1C and 1D respectively illustrate control of indirect exciton energy by gate voltage for the devices having the FIGS. 1A and 1B band diagrams. FIG. 1E illustrates control of the recombination rate of indirect excitons by gate voltage.

The generation of an indirect exciton is as has been demonstrated in coupled quantum well structures. As an example, change of an exciton emission rate by 10⁴ times as well as change of the exciton energy by 40 meV at gate voltage V=1.6 V have been previously demonstrated. See, Butov, et al, “Photoluminescence Kinetics of Indirect Excitons in GaAs/AlGaAs Coupled Quantum Wells,” Phys. Rev. B 59, 1625 (1999). The separation is achieved by an electric field in the structure growth direction and the electric field is controlled by applied gate voltage throughout the bulk material.

In the present invention, a localized gate electrode supplies electric field to provide electronic processing. Operation principles of general electronic devices are based on electron energy control by gates. In devices of the invention, energy of indirect excitons is also controlled by a gate voltage: Applied gate voltage V leads to a linear shift of the indirect exciton energy δE˜Vd as shown in FIGS. 1C-1D. Therefore, for electronic devices, indirect excitons can be used as well as electrons. In contrast to electrons, indirect excitons are optically active. They emit photons at recombination and absorb photons at photo excitation with the recombination rate and absorption coefficient being controlled electronically by gate voltage within several orders of magnitude. Thus, indirect excitons are both optically active and electronically controlled. Embodiments of the invention use this principle to provide integrated optoelectronic circuits with indirect excitons.

FIG. 2 illustrates a preferred embodiment gate controlled device of the invention, which can be referred to as an optoelectronic transistor. The device can process excitons, and can process separated holes and electrons. Transistor-like processing of excitons will be discussed first.

In these operations, an exciton is controlled between and optical input gate electrode 10 and an optical output gate electrode 12 by a control gate electrode 14. The region of the optical input gate electrode 10 can be considered an optical source, and the region of the optical output drain 12. A quantum well structure 16, which may contain a single wide quantum well, coupled quantum well, multiple quantum wells, or multiple coupled quantum wells, is separated from the gates 10-14 and a substrate 18, such as a doped conducting layer, by intrinsic semiconductor material 20. The substrate 18 can be, for example, a conducting layer: doped substrate, doped epitaxial semiconductor layer, or metal electrode.

The gates 10-14 are localized, being disposed over separate regions of the quantum well structure 16. The coupled or wide quantum well structure can have, for example, the materials and band structure of FIG. 1A or FIG. 1B.

In operation of the indirect exciton transistor of FIG. 2, excitons are photo excited in the source region of the optical input gate electrode 10 by photon absorption. Excitons travel to the drain region of the optical output gate electrode 12 under control of the control gate electrode 14. The control gate electrode 14 controls an energy barrier for indirect excitons and, therefore, controls the exciton flow. Application of a voltage pulse to the optical output gate electrode 12 increases the exciton recombination rate by several orders of magnitude thus quickly converting excitons to photons producing an optical readout in the drain region of the optical output gate. In preferred embodiments, the optical input gate electrode 10 and output gate electrode 12 are metals formed thin enough to be substantially transparent, or constitute a substantially transparent conductor, such as indium tin oxide. They can be also nontransparent; In this case optical input and output are located between the gates 10 and 14 and 14 and 12, respectively.

The indirect excitons have very long lifetimes and can travel over distances exceeding hundred of microns before recombination, i.e., over distances that are much larger than the size of the transistor device. Therefore indirect excitons travel between optical input and optical output without recombination and recombine only at optical drain region of the optical output gate electrode 12. Also, in the FIG. 2 device, the exciton energy is a linear function of gate voltage.

A result of operation cycle of the FIG. 2 devices is that the intensity of light emitted at optical output drain is proportional to intensity of light at optical input and is controlled electronically by the control gate electrode 14. Transistor-like electronic operation is accomplished with photon input and output, using excitons as intermediate media.

Optoelectronic Device Integrated Circuits

Instead of emitting photons from the top of the device as illustrated in FIG. 2, artisans will appreciate that waveguides can readily be integrated in the FIG. 2 structure to form integrated circuits. Additionally, waveguide structures can be formed on top of individual transistor devices, to couple devices. Vertical and horizontal integrations can be achieved, as will be recognized by artisans.

FIG. 3 schematically illustrates an example of integrated circuit device of the invention, including a two dimensional array of exciton circuits with many optical inputs and outputs and elaborated gate architecture form an exciton integrated circuit.

In FIG. 3, the integrated circuit has two optical inputs A and B and two optical outputs C and D, and implements electronic operations on photons including switching, sum operation, etc: The intensities of input and output lights are related as C=a₁A+b₁B and D=a₂A+b₂B with the coefficients a₁, a₂, b₁, and b₂ controlled electronically by the Gates G1-G4 via exciton flow control as illustrated with the individual device of FIG. 2.

Electron and Hole Processing

The 1A-1E/FIG. 2A devices and integrated circuits of such devices can be operated to separately process holes and electrons. An applied electric field separates electrons and holes in different quantum wells (or on different sides of a single quantum well). The devices have the same structures as described in FIGS. 1A-1E and FIG. 2. To create electron and hole processing, the optical gate electrode is controlled initially to separate electrons and holes.

Separate hole and electron processing transistor operation will be described with respect to FIG. 2A and FIGS. 4A-4E. In FIG. 4A, electrons and holes are created at the optical input gate between electrodes 10 and 14 by photon absorption. In FIG. 4B, application of voltage at the optical input gate electrode 10 separates electrons and holes in-plane. In FIG. 4C, voltage is applied to the control gate electrode 14, and moves the electrons and holes toward the region of the optical output gate electrode 12. In FIG. 4D, the voltage is turned off at the optical input gate, which moves holes further toward the region of the optical output gate electrode 12. In FIG. 4E, voltage is turned off at the control gate, which enables electrons and holes to meet at the optical output gate electrode region, which causes an intense flash of light from the region of the optical output gate electrode 12 due to electron-hole recombination.

A result of the electron and hole processing cycle of FIGS. 4A-4E is that the intensity of light emitted at optical output is proportional to intensity of light at optical input and is controlled electronically by the gate electrode 14. Transistor-like electronic operation on photons using electrons and holes as intermediate media is accomplished.

In preferred integrated circuits for photon induced hole and electron processing and photon output, there is a separate hole and electron processing circuits, as illustrated in FIG. 5. In FIG. 5, electrons and holes are created at the region of the optical input gate electrode 22 by photon absorption. In this case, the optical input gate electrode 22 (and the optical output gate electrode 24) have separate positive and negative electrodes 22 a, 22 b, 24 a, 24 b. Voltage on the optical input gates separates electrons and holes in-plane of a chip under the positive and negative electrode, respectively. Electronic operations are performed separately with electrons in an electron circuit 26 in the chip and with holes in a hole circuit 28. Typical electronic devices can be used. Photon emission from region of the optical output gate electrode 24 is controlled by voltage on the optical output gates: tuning off the voltage which separates electrons and holes in-plane enables them to recombine and emit light. The intensity of light emitted at optical output is proportional to intensity of light at optical input and is controlled electronically by gates in the electron and hole circuits.

The exciton, electron and hole circuits may be connected to many optical inputs and outputs and other circuits in a complex integrated optoelectronic circuit. Devices of the invention are scalable and form monolithically integrated optoelectronic circuits with parallel processing and many optical inputs and outputs.

Processing with Electron and Hole Spins

Electron and hole spins also have the potential for electronic effects. Spin devices have potential advantages in speed, dissipation, and size over conventional charge devices. Adding spin degree of freedom to charge-based devices can provide advantages in speed, dissipation, and size over conventional devices. The coupled or wide quantum well localized control gate electrode structures of FIGS. 1-5 can also be controlled to utilize the spin degree of freedom in optoelectronic devices.

Operation of the FIGS. 1A-5 devices can take advantage of the spin degree of freedom. In a basic optoelectronic material, e.g., GaAs, there are two optically active types of electron-hole pairs: one of them has the electron and hole spins (e+1/2, h−3/2) and emits/absorbs left-hand circularly polarized light and another pair has spins (e−1/2, h+3/2) and emits/absorbs right-hand circularly polarized light [the pairs with (e+1/2, h+3/2) and (e−1/2, h−3/2) are optically inactive]. For an electron-hole pair, the electron spin relaxation time is determined by the overlap between the electron and hole wave functions. In coupled quantum well structures the overlap between the electron and hole wave functions can be controlled electronically within more than two orders of magnitude by a moderate gate voltage V<1.6 V. Therefore, the coupled quantum well devices with localized control gates of the invention give a unique opportunity for ultrafast (GHz) electronic control of the exciton and electron spin relaxation time within more than eight orders of magnitude. This property is utilized in the invention in spin-opto-electronic processing according to the invention.

General operation principles of exemplary embodiment devices of the invention include a number of steps. A first step includes converting polarized photons into spatially separated spin-polarized electrons and holes. Another step includes performing electronic operations with spin-polarized electrons and holes. After electronic operations, spin-polarized electrons and holes are converted into polarized photons. A result of operation of the exemplary embodiment device is that the polarization of light emitted at optical output is proportional to polarization of light at optical input and is controlled electronically by gates. The device implements ultrafast electronic operation on photon polarization using electron spins as intermediate media. Example operations include switching, modulation, logical operations, etc.

With reference to FIG. 2A, spin control is used in a preferred embodiment by creation of spin-polarized excitons. This is achieved with circularly polarized light in the region of the input gate electrode 10. The circularly polarized light can be, for example, an input signal from the output of another spin-optoelectronic-transistor. Operation then proceeds as before, with the spin-polarized excitons traveling to the optical output gate electrode 12 with the control gate electrode 14 controlling the spin flow by controlling the energy. Electron spin relaxation time is long due to electron-hole separation. A voltage pulse applied to the optical output gate electrode 12 increases the exciton recombination rate thus converting the exciton spin state to the photon polarization and resulting to optical readout of the exciton spin state at the optical output.

Using polarized light input in the other processes and integrated devices described above also produces spin control devices and operations. The very long lifetimes and spin relaxation times of the excitons in coupled or wide quantum wells allows transferring spin coherence over the device from optical input to optical output. A short spin relaxation time for holes does not reduce degree of the photon polarization at the optical output since holes (for example in GaAs) become optically inactive after spin-flip. (Indeed, for an optically active pair (e+1/2, h−3/2) created at optical input, after the hole spin relaxation, the pair (e+1/2, h+3/2) becomes optically inactive and cannot emit photons at the photonic output.)

While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.

Various features of the invention are set forth in the appended claims. 

1. An optical interface electrically controlled device, comprising: photonically responsive intrinsic semiconductor material upon a substrate; a quantum well structure disposed within said photonically responsive intrinsic semiconductor material, the quantum well structure being dimensioned, arranged and formed from materials that create a nonzero distance d between a separated electron and hole; an input gate electrode disposed over an input region of said photonically responsive intrinsic semiconductor material and said quantum well structure; an output gate electrode disposed over an output region of said photonically responsive intrinsic semiconductor material and said quantum well structure; and a localized control gate electrode disposed over a control gate electrode region of said photonically responsive intrinsic semiconductor material to control electrical flow between said input region and said output region.
 2. The device of claim 1, wherein said quantum well structure comprises two quantum well layers separated by a thin barrier layer.
 3. The device of claim 2, wherein said quantum well structure comprises two quantum well layers of different materials.
 4. The device of claim 1, wherein said quantum well structure comprises a wide single quantum well that create a nonzero distance d between a separated electron and hole.
 5. The device of claim 1, wherein each of said input and output gate electrodes are biased relative to the substrate electrode.
 6. The device of claim 4, further comprising separate hole and electron processing circuits.
 7. An integrated electro optical circuit including a plurality of devices according to claim
 1. 8. A method of processing excitons with a device according to claim 1, the method comprising steps of: photo exciting excitons in said input region with a voltage applied to said input gate electrode; controlling exciton flow to said output region with voltage applied to said control gate electrode; and converting excitons to photons in said output region by application of a voltage pulse to said optical output gate electrode.
 9. The method of claim 8, wherein said step of photo-exciting comprises producing spin polarized excitons with polarized light.
 10. A method of separately processing holes and electrons with a device according to claim 1, the method comprising steps of: creating electrons and holes at the input region photon absorption under with a voltage applied to said input gate electrode; moving electrons and holes toward the output region with voltage applied to the control gate electrode; moving holes further toward the output region by removing voltage from the input gate electrode; permitting electrons and holes to meet in the output region by removing voltage from the control gate electrode.
 11. The method of claim 10, wherein said step of photo-exciting comprises producing spin polarized electrons and holes with polarized light. 